1. Field of the Invention
The present invention relates to a polycrystalline translucent alumina sintered body having an excellent in-line transmission, a method for producing the same and a high pressure vapor discharge lamp obtained by using said alumina sintered body. The polycrystalline translucent alumina has excellent light transmission, heat resistance, corrosion resistance and the like, so that such an alumina sintered body has been broadly used for an envelope of a high pressure vapor discharge lamp, window for high temperature, memory erasing window and the like.
2. Description of the Prior Art
As a method for producing such polycrystalline translucent alumina, it has been known that about 0.1% of MgO is added to fine, high purity alumina powder as an additive to control exaggerated grain growth of alumina crystal grains, whereby a dense polycrystalline translucent alumina sintered body having substantially no pores is produced. However this method has had the defect that firing at a high temperature of higher than 1,800.degree. C. is necessary for obtaining a sintered body having an excellent transmission. A process wherein about 0.05-0.1% of each of Y.sub.2 O.sub.3 and La.sub.2 O.sub.3 is added as an additive in addition to MgO has been disclosed but even though the firing temperature is lowered to 1,650.degree.-1,750.degree. C. owing to the function of Y.sub.2 O.sub.3, La.sub.2 O.sub.3 and the like, the effects of these additives influencing the grain growth of alumina particles are different. Thus, alumina crystal grains in the obtained sintered body are apt to be grown nonuniformly depending upon the ratio of MgO, La.sub.2 O.sub.3 and Y.sub.2 O.sub. 3 added and therefore it cannot be avoided to degrade the mechanical strength and thermal shock resistance. Furthermore, since amounts of MgO, La.sub.2 O.sub.3, Y.sub.2 O.sub.3, etc., added are larger, different phases are formed in the crystal grain boundary and the in-line transmission is very poor and there have been many defects.